1. Field of the Invention
The present invention relates to a medical imaging apparatus for imaging mammary gland and breast by using radiation and ultrasonic waves for diagnoses of breast cancer.
2. Description of a Related Art
Conventionally, an imaging method using radiation (X-ray, α-ray, β-ray, γ-ray, electron ray, ultraviolet ray, or the like) is utilized in various fields, and particularly, in the medical field, the method is one of the most important means for diagnoses. Radiation images obtained by X-ray imaging (X-ray mammography) of breasts for breast cancer diagnoses are useful for finding calcification as a precursor of mass and cancer, but finding calcification may be difficult depending on mammary gland density or the like of an object to be inspected. Accordingly, it has been studied to use radiation and ultrasonic waves in combination to make diagnoses based on both radiation images and ultrasonic images. X-ray mammography and ultrasonic imaging have the following features, respectively.
X-ray mammography is suitable for exposing calcification as one of early symptoms of a cancer, and enables detection with high sensitivity and high resolving power. Especially, in the case where mammary gland tissues have become atrophied and replaced with fat (so-called “fat breast”) as is the case of postmenopausal women, more information can be obtained by X-ray mammography. However, the X-ray imaging has a disadvantage that detection capability of specific natures of tissues (tissue properties) is low.
Further, in an X-ray image, mammary glands are expressed in homogeneous soft tissue density, and thus, the detection of tumor mass is difficult for the case where mammary glands have developed (so-called, “dense breast”) as is the case of adolescent to premenopausal women. Furthermore, in X-ray mammography, only two-dimensional images can be obtained in which an object to be inspected as a solid is projected on a plane. On this account, even when a tumor mass is found, it is difficult to grasp information on the location in the depth direction, size, and so on of the tumor mass.
On the other hand, in ultrasonic imaging, specific natures of tissues (e.g., the difference between a cystic tumor and a solid matter) can be detected and also a lobular cancer can be detected. Further, real time observation of images and three-dimensional image generation are possible. However, accuracy of ultrasonic imaging examination often depends on the skill of an operator such as a doctor, and reproducibility is low. Further, it is difficult to observe minute calcification in an ultrasonic image.
As described above, X-ray mammography examination and ultrasonic imaging examination have both merits and demerits, and it is desirable that both examinations are performed for reliably finding breast cancer. Since the X-ray mammography examination is performed while the object (breast) is compressed by a compression plate, in order to make diagnoses based on X-ray images and ultrasonic images of the object in the same condition, the ultrasonic imaging examination is necessary to be performed in the same condition as that when the X-ray mammography examination is performed, that is, while the object (breast) is compressed by the compression plate. For this purpose, a medical imaging apparatus for imaging mammary gland and breast by using radiation and ultrasonic waves in combination is considered.
In the medical imaging apparatus, when the object (breast) compressed by the compression plate is mechanically scanned by using a one-dimensional ultrasonic transducer array, there is a problem that it takes time to obtain an ultrasonic image. Accordingly, in order to reduce the time taken for obtaining an ultrasonic image, it is considered to use a two-dimensional ultrasonic transducer array.
As a related technology, Japanese Patent JP-B-P3461509 (International Publication WO95/011627) discloses an apparatus for imaging breast tissues by using both X-ray and ultrasonic technologies including a two-dimensional ultrasonic transducer array below a grid provided under a compression plate. In this apparatus, connecting wires of the two-dimensional ultrasonic transducer array are provided across the two-dimensional ultrasonic transducer array in alignment with X-ray absorbent material rows of the grid, and the connecting wires form no image on an X-ray film during radiation to the X-ray film.
However, if X-ray imaging is performed with the grid remaining stationary, no image of the connecting wires is formed but an image of the grid is formed. In order not to form the image of the grid, it is necessary to perform X-ray imaging while moving the grid with the two-dimensional ultrasonic transducer array, and the movement mechanism becomes complex in this case.